Ion mobility spectroscopy (IMS), sometimes known as plasma chromatography, is a technology that is ideally suited for the detection of very low levels of analyte due to its extreme sensitivity and ability to speciate. IMS is widely used to detect narcotics, explosives, and chemical warfare agents, since the technique can be tailored to be particularly sensitive to compounds that form negative ions, such as nitrate-laden explosives. Further, because IMS can operate at atmospheric pressure and can detect trace quantities of explosives, it is an attractive technology for use in a miniaturized explosives sensor. See L. M. Matz et al., Talanta 54, 171 (2001); C. Wu et al., Talanta 57, 123 (2002); M. Tabrizchi et al., Int. J. Mass Spectrom. 218, 75 (2002); and K. Cottingham, Anal. Chem. 75, 435A (2003).
Ion mobility spectroscopy is based on the atmospheric pressure ionization of a sample vapor and the subsequent separation of the individual ionized components of the sample mixture via electrophoresis as they are accelerated by an external electric field gradient and transit a time-of-flight drift tube against a neutral, counter-flowing gas stream. See G. A. Eiceman and Z. Karpas, Ion Mobility Spectrometry, 2nd Ed., Chapter 4, CRC Press, Boca Raton, Fla., (2004).
FIG. 1 shows a schematic illustration of a conventional ion mobility spectrometer 10. The sample vapor 12 is drawn into an IMS drift tube 21 and ionized in an ionization region 22 (e.g., using a radioactive source, photoionization, or corona discharge ionizer 23), typically through proton transfer or electron capture reactions with reactant ions, to form product ions. The direction of travel of the ions depends on the polarity of the electric field 24. For example, common explosives contain electronegative nitro functional groups. Therefore, the ionization chemistry for explosives tends to form negative ions. Halogenated compounds, such as methylene chloride, can be added to a carrier gas in the ionization region 22 to provide chloride reactant ions (i.e., Cl−). The chloride reactant ions can then transfer charge to the electronegative explosive molecules to form molecular ions.
A gate drive circuit 25 provides a trigger to a gate 26, thereby providing an ion pulse 14 that is gated into a drift region 15 to begin a new measurement cycle. IMS drift tubes have normally been operated by opening an electrostatic ion shutter to allow a narrow pulse of ions into the time-of-flight drift region where they move toward an ion collector 27 as a single ion swarm to be measured as a transient collected current. The gate can be an electrostatic ion shutter, such as a Bradbury-Neilson or Tyndall type shutter. With a Bradbury-Neilson shutter, a transverse electric field is applied to drive the ions into a perpendicular trajectory from the axis into a conductor where ion annihilation occurs resulting in a cutoff of ion flow in the drift tube. The related Tyndall shutter uses two closely spaced planes of electrodes consisting of parallel wires or screens. A voltage is applied or removed between the planes to annihilate the ions during an off cycle and then release them into the drift tube as a pulse of ions. Conventional gating techniques, such as Tyndall and Bradbury-Neilson gates, operate more like camera shutters and do not compress the ions as a result of an accumulation cycle. Another type of gate uses a single potential plane to form a potential well in the drift tube. This gate provides a potential capture well that controls the injection of ions into the drift region by first collecting the ions and then releasing them as a pulse.
Drift gas is injected into the drift tube 21 via a drift gas inlet 17 and removed through a drift gas outlet 18. In the drift region 15, the ions establish a terminal velocity under the influence of the potential gradient of the electric field 24 and are separated into single ion swarms according to their characteristic ion mobility against the counter-flowing drift gas 16. The separation begins at the entrance gate 26 and terminates at the ion collector 27 at the end of the drift region 15, where the ion response signal is recorded. For example, the ion collector 27 can comprise a collecting electrode or Faraday plate that records an ion response current. The response of the IMS drift tube 21 is measured as a function of ion current versus the ion arrival time at the ion collector 27 for a measurement cycle. Typically, the ion detector comprises an operational amplifier 28 for converting the ion response current into an ion mobility spectrum. The spectrum of ion arrival times at the ion collector indicates the relative ion mobility of each ion through the drift region. Compound identification is typically based on the comparison of the ion mobility spectrum generated from the sample with the spectrum of a known standard.
The resolution of the IMS is related to the drift time divided by the pulse width at the one-half amplitude of a single ion swarm. The pulse width is subject to several broadening mechanisms including the initial gated pulse width, diffusional broadening, electrostatic space charge repulsion, electric field gradients, temperature gradients, gate depletion/dynamic leakage, pressure fluctuations, ion molecule reactions in the drift space, and capacitive coupling between approaching ions and the ion collector. See R. H. St. Louis and H. H. Hill, Anal. Chem. 21, 321 (1990). In particular, capacitive coupling between an approaching ion swarm and the ion collector causes an asymmetry in the rising edge of the response current. Therefore, a physical aperture grid 29, constructed from an array of small wires suspended across the interior of the drift tube, is located just ahead of the ion collector 27 to capacitively decouple the approaching ion cloud and prevent peak broadening due to a premature detector response. However, the addition of an aperture grid to the spectrometer results in increased complexity of the system in both assembly and function. In addition, it leads to a large source of noise in the spectrometer as the grid can be vibration sensitive and generate an additional current that is a function of acoustic vibration in the environment.
Therefore, a need remains for an ion mobility spectrometer that does not require a physical aperture grid to eliminate capacitive coupling of an ion swarm to the ion collector.